A schematic representation of part of the human auditory system is shown in FIG. 9. The auditory system is generally comprised of an external ear AA, a middle ear JJ, and an internal ear FF. The external ear AA includes the auditory canal BB and the tympanic membrane CC, and the internal ear FF includes an oval window EE and a vestibule GG which is a passageway to the cochlea (not shown). The middle ear JJ is positioned between the external ear and the middle ear, and includes a eustachian tube KK and three bones called ossicles DD. The three ossicles DD: the malleus LL, the incus MM, and the stapes HH, are positioned between and connected to the tympanic membrane CC and the oval window EE.
In a person with normal hearing, sound enters the external ear AA where it is slightly amplified by the resonant characteristics of the auditory canal BB of the external ear. The sound waves produce vibrations in the tympanic membrane CC, part of the external ear that is positioned at the proximal end of the auditory canal BB. The force of these vibrations is magnified by the ossicles DD.
Upon vibration of the ossicles DD, the oval window EE, which is part of the internal ear FF, conducts the vibrations to cochlear fluid (not shown) in the inner ear FF thereby stimulating receptor cells (not shown), or hairs, within the cochlea. In response to the stimulation, the hairs generate an electrochemical signal which is delivered to the brain via one of the cranial nerves and which causes the brain to perceive sound.
A number of auditory system defects will impair or prevent hearing. Some patients have ossicles that lack the resiliency necessary to increase the force of vibrations to a level that will adequately stimulate the receptor cells in the cochlea. Other patients have ossicles that are broken, and which therefore do not conduct sound vibrations to the oval window.
Prostheses for ossicular reconstruction are sometimes implanted in patients who have partially or completely broken ossicles. These prostheses are normally cut to fit snugly between the tympanic membrane CC and the oval window EE or stapes HH. The close fit holds the implants in place, although gelfoam is sometimes packed into the middle ear to ensure against loosening. Two basic forms are available: total ossicle replacement prostheses (TORPs), which are connected between the tympanic membrane CC and the oval window EE; and partial ossicle replacement prostheses (PORPs), which are positioned between the tympanic membrane and the stapes HH.
Although these prostheses provide a mechanism by which vibrations may be conducted through the middle ear to the oval window of the inner ear, additional devices are frequently necessary to ensure that vibrations are delivered to the inner ear with sufficient force to produce high quality sound perception. Even when a prosthesis is not used, disease and the like can result in hearing impairment.
Various types of hearing aids have been developed to restore or improve hearing for the hearing impaired. With conventional hearing aids, sound is detected by a microphone, amplified using amplification circuitry, and transmitted in the form of acoustical energy by a speaker or transducer into the middle ear by way of the tympanic membrane. Often the acoustical energy delivered by the speaker is detected by the microphone, causing a high-pitched feedback whistle. Moreover, the amplified sound produced by conventional hearing aids normally includes a significant amount of distortion.
Attempts have been made to eliminate the feedback and distortion problems associated with conventional hearing aid systems. These attempts have yielded devices which convert sound waves into electromagnetic fields having the same frequencies as the sound waves. A microphone detects the sound waves, which are both amplified and converted to an electrical current. The current is delivered to a coil winding to generate an electromagnetic field which interacts with the magnetic field of a magnet positioned in the middle ear. The magnet vibrates in response to the interaction of the magnetic fields, causing vibration of the bones of the middle ear or the skull.
Existing electromagnetic transducers present several problems. Many are installed using complex surgical procedures which present the usual risks associated with major surgery and which also require disarticulating (disconnecting) one or more of the bones of the middle ear. Disarticulation deprives the patient of any residual hearing he or she may have had prior to surgery, placing the patient in a worsened position if the implanted device is later found ineffective in improving the patient's hearing.
Existing devices also are incapable of producing vibrations in the middle ear which are substantially linear in relation to the current being conducted to the coil. Thus, the sound produced by these devices includes significant distortion because the vibrations conducted to the inner ear do not precisely correspond to the sound waves detected by the microphone.
An easily implantable electromagnetic transducer is therefore needed which will conduct vibrations to the oval window with sufficient force to stimulate hearing perception and with minimal distortion.